Geomorphologic and mineralogic characterization of the northern plains of Mars at the Phoenix Mission candidate landing sites

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113,, doi: /2008je003088, 2008 Geomorphologic and mineralogic characterization of the northern plains of Mars at the Phoenix Mission candidate landing sites K. D. Seelos, 1 R. E. Arvidson, 2 S. C. Cull, 2 C. D. Hash, 3 T. L. Heet, 2 E. A. Guinness, 2 P. C. McGuire, 2 R. V. Morris, 4 S. L. Murchie, 1 T. J. Parker, 5 T. L. Roush, 6 F. P. Seelos, 1 and M. J. Wolff 7 Received 29 January 2008; revised 5 May 2008; accepted 3 June 2008; published 11 September [1] A suite of remote sensing data is used to evaluate both geomorphology and mineralogy of the candidate landing sites for the 2007 Phoenix Mission. Three candidate landing site boxes are situated in the northern plains of Mars on the distal flank of Alba Patera in the region from 67 Nto72 N and from 230 E to 260 E. Geomorphology is mapped at subkilometer spatial scales using Thermal Emission Imaging System (THEMIS) visible and Mars Orbiter Laser Altimeter (MOLA) topographic data, supplemented by images from the High-Resolution Imaging Science Experiment (HiRISE) and Context Imager (CTX). Mineralogy and spectral properties are examined using Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) visible and near-infrared multispectral mapping and targeted hyperspectral data at 200 and 20 m/pixel, respectively. Geomorphic mapping supports the idea that terrains along the boundary between the Amazonian Scandia region and Vastitas Borealis marginal geologic units have undergone extensive modification. Intercrater plains are disrupted to form mesas and interlocking blocks, while irregular depressions and knobby terrain are consistent with erosion/subsidence and local deposition. Despite the varied morphology, the present-day surface is nearly homogeneous with spectral signatures dominated by nanophase iron oxides and basaltic sand and rocks, similar to that of the Gusev crater plains at the Mars Exploration Rover (MER) landing site. The compilation of geomorphic and spectral information for the candidate Phoenix landing sites provides a framework for the mission s in situ observations to be extrapolated to the northern plains as a whole. Citation: Seelos, K. D., et al. (2008), Geomorphologic and mineralogic characterization of the northern plains of Mars at the Phoenix Mission candidate landing sites, J. Geophys. Res., 113,, doi: /2008je Introduction [2] Over the last four decades of Mars exploration, two landers and three rovers have successfully investigated a variety of equatorial and midlatitude terrains where geologic, morphologic, and/or mineralogic evidence indicates the past presence of water [Mutch et al., 1977; Golombek et al., 1999; Squyres et al., 2004a, 2004b]. Morphologic evidence for present-day water at lower latitudes has been observed [Malin and Edgett, 2000], but most volatile activity today remains associated with the polar caps [e.g., Piqueux et al., 1 Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA. 2 Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri, USA. 3 Applied Coherent Technology Corporation, Herndon, Virginia, USA. 4 NASA Johnson Space Center, Houston, Texas, USA. 5 NASA Jet Propulsion Laboratory, Pasadena, California, USA. 6 NASA Ames Research Center, Moffett Field, California, USA. 7 Space Science Institute, Brookfield, Wisconsin, USA. Copyright 2008 by the American Geophysical Union /08/2008JE ; Titus et al., 2007]. It is thought that the majority of Martian water resides in the subsurface, and orbital instruments have been able to detect large repositories of water ice just below the surface at high latitudes [Boynton et al., 2002; Bandfield and Feldman, 2007]. On 25 May 2008, the Phoenix Lander touched down on the northern plains of Mars to sample and characterize shallow water ice and to document the high-latitude surface and atmospheric environment in which it occurs [Smith, 2006; Arvidson et al., 2008]. Results from the Phoenix experiment will have widespread implications for understanding the current climate, habitability potential, and interactions between subsurface, surface, and atmospheric constituents. [3] In this paper we summarize the geomorphology and mineralogy of the candidate Phoenix landing sites to provide context and regional applicability for the in situ measurements to be conducted during the mission. A number of remotely sensed data sets acquired by the Mars Global Surveyor (MGS), Mars Odyssey (ODY), and Mars Reconnaissance Orbiter (MRO) missions are utilized in this effort. Thermal Emission Imaging System (THEMIS) visible data [Christensen et al., 2004a] together with Mars 1of16

2 Orbiter Laser Altimeter (MOLA) topography [Zuber et al., 1992] provide a basis for geomorphologic mapping. Where available, higher spatial resolution Context Imager (CTX) [Malin et al., 2007] and High Resolution Imaging Science Experiment (HiRISE) [McEwen et al., 2007] data are used to provide morphologic information on the scale of meters to tens of meters. Mineralogic variability is determined through analysis of Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) visible and near-infrared reflectance data [Murchie et al., 2007]. The two operational modes of CRISM (multispectral 200 m/pixel and hyperspectral 20 m/pixel) provide both regional coverage and high-resolution coverage for select areas. Together, surface spectral properties in the context of mapped geomorphologic units provide an informative framework in which Phoenix observations may be placed and extrapolated to the northern plains region at large. 2. Background 2.1. Phoenix Overview and Landing Site Selection [4] A comprehensive description of the Phoenix Lander, instrument payload, science goals and objectives, and landing site selection process and status is given by Smith et al. [2008] and Arvidson et al. [2008]. An abbreviated overview is presented here. [5] Phoenix is a Scout class mission focused on the follow the water theme of Mars exploration. Specifically, Phoenix will land at a high northern latitude site to test the hypothesis that water ice exists only centimeters beneath the surface and to evaluate its habitability potential [Smith et al., 2008]. The instrument payload is optimized for achieving these goals as well as the parallel objectives of characterizing the expected periglacial landforms and monitoring the high-latitude atmospheric dynamics. Of special note is the 4-degree-of-freedom, 2.35-m-long robotic arm (RA) that is designed to dig up to 50 cm into the substrate [Bonitz et al., 2001] and deliver samples of trenched materials to the Thermal and Evolved Gas Analyzer (TEGA) [Boynton et al., 2001] and the Mars Environmental Characterization Analyzer (MECA) [Kounaves et al., 2003]. Together, these instruments will determine the mineral and volatile constituents, ph, microscopic structure, and conductivity of the retrieved soil and volatile samples [Boynton et al., 2001; Kounaves et al., 2003]. The RA is also outfitted with the Robotic Arm Camera (RAC) to monitor the trench site(s) and samples within the scoop with high-resolution, fullcolor images [Bonitz et al., 2001] and a Thermal and Electrical Conductivity Probe (TECP) [Zent et al., 2008] to determine soil and ice properties and to measure atmospheric relative humidity. At the same time, the Surface Stereo Imager (SSI) will acquire stereo views of the landing site, surrounding terrain, and trenching operations [Smith et al., 2001]. The SSI resides on a mast roughly 2 m above the ground surface and contains 13 visible and near-infrared narrow band pass filters to identify geologically interesting materials. Looking skyward, the SSI will provide information on the optical properties and aerosol components of the atmosphere. The Meteorological Station (MET) will also supply information about atmospheric dust and ice aerosols by using a light detection and ranging (LIDAR) device, and will monitor daily fluctuations in near-surface temperature and pressure. Stereo SSI images will also be used to measure the displacement magnitude and direction of a telltale [Gunnlaugsson et al., 2008] to record wind speed and direction. [6] A key science objective of the Phoenix mission is to sample and analyze both dry surface soils and subsurface ice-rich soils [Smith et al., 2008]. To that end, it is necessary to choose a landing site where both materials are present and accessible to the RA, while also respecting engineering and safety constraints. Entry, decent, and landing (EDL) requirements constrain the location of the landing site to be from 65 N to 72 N and below-3500 m elevation with respect to the MOLA-defined areoid [Guinn et al., 2006; Arvidson et al., 2007, 2008]. In addition, lander-scale slopes must be less than 16 [Kirk et al., 2008] and rock abundance values below 18% [Golombek et al., 2008], similar to the Viking 2 site located west of Mie crater at approximately 48 N, 134 E [Mutch et al., 1977]. [7] Down-selection of candidate landing sites has occurred through a series of workshops leading up to the August 2007 launch date [Guinn et al., 2006]. Because the northern plains are largely homogeneous, an early decision was to focus orbital observation resources by choosing four 20 (in longitude) regions within the N latitudinal band that were devoid of obvious hazards and that exhibited a breadth of inferred dry layer thickness (DLT) to allow sampling of both dry surface soils and water ice-rich subsurface soil. DLT (in g/cm 2 ) is a modeled parameter derived from the analysis of Gamma Ray Spectrometer (GRS) data [Boynton et al., 2006; Mellon et al., 2008] and yields an estimate of the thickness of the dry soil layer overlying a water ice-rich layer, given values for soil density and percentage of pore space occupied by ice. Additional evaluations of ice depth using Thermal Emission Spectrometer (TES) and THEMIS infrared data are consistent with the results from GRS [Bandfield and Feldman, 2007; Titus and Prettyman, 2007]. The four regions were therefore chosen across a range of DLT values suitable to maintain accessibility to both dry and ice-rich soils: region A (low DLT) spans the longitude range from 250 to 270 E, region B (intermediate DLT) from 120 to 140 E, region C (high DLT) from 65 to 85 E, and region D (very low DLT) from 230 to 250 E [see Arvidson et al., 2008, Figure 2]. [8] Subsequent workshops utilized newly acquired THEMIS and Mars Orbital Camera (MOC) data to evaluate and prioritize the four regions, with region B selected as the preferred location primarily because of its low elevation and allowance for greater EDL margin [Arvidson et al., 2008]. In the fall of 2006 and with the onset of the MRO primary science data acquisition phase [Zurek and Smrekar, 2007], the unprecedented spatial resolution (31 cm/pixel) of the HiRISE instrument revealed widely distributed boulder fields that ended consideration of region B as a viable landing site [Golombek et al., 2008]. Examination of HiRISE images elsewhere in the Phoenix latitudinal band quickly showed that regions A and D exhibited much lower and acceptable rock abundances [Golombek et al., 2008]. As a result, three 5 latitude by 9 longitude candidate landing site boxes were selected from within the adjacent regions A and D. Box 1 is centered at N, E, box 2 is at N, 247 E, and box 3 is at N, 253 E [Arvidson et al., 2008]. These boxes circumscribe all 2of16

3 possible landing error ellipse azimuth orientations and axes dimensions, which vary depending on the launch date. After a successful launch on 4 August 2007, the primary 3-sigma landing ellipse is centered within box 1 at N, E, with an azimuth of 110 and axes lengths of 106 and 21 km. This paper retains focus on all three candidate sites, however, in order to provide localized evaluations of three broadly similar northern lowland areas Northern Plains Geology [9] The candidate Phoenix landing sites are situated on the distal northern flank of Alba Patera in Borealis basin, with regional slopes on the order of 0.1 at kilometer baselines tilted toward the north (Figure 1). To the south of the landing sites, fractured Hesperian-aged volcanic plains are embayed by younger materials of the Vastitas Borealis marginal (ABv m ) and Scandia region (ABs) geologic units [Tanaka et al., 2005]. The Scandia region unit extends westward and northward to the circumpolar Olympia Undae dune field and north polar plateau. To the east, the Vastitas Borealis marginal unit transitions to the Vastitas Borealis interior unit, which occupies a majority of the northern plains [Tanaka et al., 2005]. [10] The three landing sites reside on the Vastitas Borealis marginal unit and the Scandia region unit. The marginal unit of the Vastitas Borealis Formation is described by Tanaka et al. [2005] as a smooth plains forming unit with low plateaus, sometimes dissected by shallow sinuous troughs. Together with the hummocky, sometimes polygonally fractured Vastitas Borealis interior unit (ABv i ), these terrains define the beginning of the Amazonian Period of Mars history. The Vastitas Borealis units are interpreted to be sedimentary deposits originating from highland outflow channels and augmented by locally derived secondary materials. The marginal unit (ABv m ) occupies the outer high-elevation portions of the Vastitas Borealis Formation and is gradational with the interior unit (ABv i ). It is thought that deformation and reworking of the deposits by periglacial and other near surface processes may have contributed to current morphology, with reworked materials comprising the present surface of both units. [11] The Scandia region unit (ABs) stratigraphically overlies the Vastitas Borealis units and is characterized by widespread knobby terrain, mantled materials, and irregular topographic depressions. The varied morphology and extensive deformation of the Scandia region unit is thought to have been caused by subsurface volatile activity leading to mud volcano-like processes and/or cryoclastic eruptions, possibly related to intrusive activity and enhanced heat flow sourced to Alba Patera [Tanaka et al., 2005]. Despite the stratigraphic relationship, the Scandia terrains are often topographically lower than nearby outcrops of the Vastitas Borealis units, perhaps as a result of multiple episodes of reworking and deformation. 3. Geomorphologic Characterization 3.1. Data Set and Method Description [12] Two primary data sets were utilized in characterizing the geomorphology of the candidate landing sites: THEMIS visible data and MOLA altimetry. These particular data were chosen because they offer relatively high spatial resolution and exhibit nearly complete aerial coverage. Higher-resolution CTX and HiRISE data were also examined where available, usually near landing ellipse centers. THEMIS, CTX, and HiRISE data provide apparent albedo contrast that allows for delineation of small terrain elements and boundaries between units, while MOLA topographic data yield relief and slope properties. [13] The THEMIS instrument is a multispectral camera with five wavelength channels in the visible ( mm) and ten in the infrared ( mm) [Christensen et al., 2004a, 2006]. THEMIS visible data used here were acquired in a single channel (band 3) centered at mm and at 18 or 36 m/pixel spatial resolution. The CTX [Malin et al., 2007] and HiRISE [McEwen et al., 2007] instruments also operate in the visible wavelengths with band passes from 0.5 to 0.7 mm and from 0.55 to 0.85 mm, respectively. Spatial resolutions are approximately 6 m/pixel for CTX and 31 cm/pixel for HiRISE. For each site, the THEMIS data were converted to a north polar stereographic map projection with longitude below center of 250 E, and mosaicked together at a resampled spatial resolution of 20 m/pixel. Colorized MOLA topographic data [Zuber et al., 1992] gridded at 256 pixel/degree (231 m/pixel) resolution were overlain at reduced opacity on the THEMIS mosaics and supplementary CTX and HiRISE data. Shaded relief products were also utilized, generated from MOLA data using an incidence angle of 45 and sun azimuth of 225 (from lower left). [14] Local, geomorphic units were therefore defined on the basis of topographic expression and spatially coherent relative albedo variations. The spatial resolution of the combined THEMIS and MOLA base maps resulted in a resolvability limit of approximately 50 m, thus no features smaller than this were mapped even though high-resolution data (CTX, HiRISE) were available for localized areas. Owing to the rapid change in landing site prioritization, the only geomorphic units mapped at this level of detail (50-m scale) were craters, crater ejecta, and the terrains within the primary landing ellipse of box 1. Most features were instead distinguished at a scale of a few hundred meters Unit Descriptions [15] Seven geomorphic units were mapped at the three candidate landing sites: highland (H), blocks/mesas (B/M), knobs (K), lowland dark (Ld), lowland bright (Lb), crater interior (Ci), and crater ejecta (Ce) (Figures 2 4). The units and their spatial distributions are described below, along with relationships to previously discussed geologic units. Closely related units are grouped together for simplicity Highland [16] The highland unit (H) comprises smooth intercrater plains with gentle undulations punctuated by knobs, depressions, mesas, and impact craters. It is recognized by a marked topographic difference relative to the lowland geomorphic unit (see below). While the topographic disparity is most evident in box 1, ranging from 100 to 200 m, the characteristic smooth plains of the highland unit are best observed in box 2. The highland unit generally correlates spatially with the Vastatas Borealis marginal unit as defined by Tanaka et al. [2005]. Although box 3 is also mapped primarily as highland, the intercrater plains here are more uneven in appearance and fall within 3of16

4 Figure 1. Regional context for the Phoenix candidate landing sites. The N latitudinal restriction is outlined in white, and candidate landing site boxes 1, 2, and 3 are labeled. (a) MOLA topography shows a gradual northward slope from Alba Patera into Borealis basin. (b) Geology from Tanaka et al. [2005] is dominated by the Scandia region unit (ABs; yellow) as well as the Vastitas Borealis marginal (ABv m ; dark green) and interior (ABv i ; light green) units. 4of16

5 Figure 2. Phoenix candidate landing site box 1 (a) MOLA topography and (b) geomorphology. Geomorphic units include highland (H), blocks/mesas (B/M), knobs (K), lowland dark (Ld), lowland bright (Lb), crater ejecta (Ce), and crater interior (Ci) (see section 3.2 for descriptions). The type area for the knobs unit is outlined in black and is shown in Figure 5b. The CTX and CRISM images detailed in Figure 6 are highlighted in red. the area identified as the Scandia region unit [Tanaka et al., 2005] Blocks/Mesas [17] Flat-topped, elevated landforms that are distinct from surrounding terrain are mapped as the blocks/mesas unit (B/M). Vertical relief is generally only a few tens of meters, although mesas in box 1 that occur alongside the valley mapped as lowland unit exhibit a combined relief of m. Individual mesas have widths of approximately 5 to 50 km, and several exhibit raised, rampart-like edges with enough relief at times to be identified as the knobs unit (see description below). Several examples of these rimmed mesas are located in box 2 within highland terrain (Figures 3 and 5a), and are semirectangular in planform. Some occurrences of the blocks/mesas terrain have smooth margins, namely two large mesas in the north portion of box 1 and numerous interlocking blocks on the south side of the valley (Figure 2). This set of mesas are mapped by Tanaka et al. [2005] as outliers of the Vastitas Borealis marginal unit, and may be evidence for disruption or deformation of the unit along its SW-NE boundary Knobs [18] Rounded, commonly clustered hills constitute the knobs unit (K; see Figure 5b). Occurrences of this unit are identified primarily on the basis of topographic expression; few albedo indicators other than illumination effects are observed. Individual hills display relief of several tens of meters to a few hundred meters, and are generally 2 5 km in basal diameter. Knob terrain is concentrated in a wide zone oriented SW-NE across the region that correlates spatially to the boundary between the Scandia region and Vastitas Borealis marginal geologic units, most prominently observed boxes 1 and 2 (Figures 2 3). However, the knobs unit does occur throughout all three boxes, typically 5of16

6 Figure 3. Phoenix candidate landing site box 2 (a) MOLA topography and (b) geomorphology. Geomorphic units include highland (H), blocks/mesas (B/M), knobs (K), lowland (L), crater ejecta (Ce), and crater interior (Ci) (see section 3.2 for descriptions). Type localities for blocks/mesas and lowland units are outlined in black and are shown in Figures 5d and 5c, respectively. The CTX and CRISM images detailed in Figure 8 are highlighted in red. grouped along the margins of shallow topographic depressions but also singly with no apparent regard to other units (e.g., as seen in Figure 8). The origin of the knobs unit does not appear to be related to craters, as there are no signs of impact structures. Instead, low albedo, linear features are occasionally observed, and in at least one case, a chain of small semicircular depressions. These caldera-and dike-like features may be consistent with volcanic activity and/or of the action of subsurface ice in the form of mud volcanoes or pingos. Because of the nearby association with the blocks/ mesas unit, it is also possible that these two classes of landforms are genetically related and knob terrain is a precursor to or a more degraded form of blocks/mesas terrain Lowland, Dark and Bright [19] Areas that are topographically depressed relative to the surrounding terrain are classified as the lowland unit (Ld, Lb; see Figure 5c). For the most part, this unit comprises shallow irregular, enclosed basins that are several tens of kilometers across and often bordered by knob terrain or a raised edge of a few tens of meters. The floors of the depressions are extremely flat with slopes well below 0.5 at 500 m length scales, and the elevation difference with the surrounding plains ranges from 50 to a few hundred meters. Lowland depressions appear to become more frequent and increase in size toward the north and west, eventually transitioning to entire valleys. These aerially extensive portions of the lowland unit coincide with what has been mapped as the Scandia region geologic unit [Tanaka et al., 2005]. [20] The valley that occupies the central portion of box 1 is the primary landing site for Phoenix (Figure 2). In this high-priority area, the lowland unit has been further delineated into bright and dark subunits the basis of relative albedo. Analysis of HiRISE data show that the dark subunit exhibits higher rock abundance than does the bright, contributing to the albedo difference [Golombek et al., 2008]. The lowland dark unit appears to drape over preexisting landforms and is also elevated by a few meters relative to the lowland bright unit. Lowland dark terrain is more commonly located in the western portion of the box 1 valley, although semicircular patches of bright terrain are 6of16

7 Figure 4. Phoenix candidate landing site box 3 (a) MOLA topography and (b) geomorphology. Geomorphic units include highland (H), blocks/mesas (B/M), knobs (K), lowland (L), crater ejecta (Ce), and crater interior (Ci) (see section 3.2 for descriptions). Type location for the crater interior and ejecta units is outlined in black and is shown in Figures 5a. The CTX and CRISM images detailed in Figure 10 are highlighted in red. interspersed and commonly coupled with small degraded craters or secondaries. The largest contiguous area of lowland bright terrain is in association with the distal ejecta deposit of a 10 km diameter crater (Heimdall) that is centered on the southeast escarpment of the valley (see also Arvidson et al. [2008]). The correlation of the bright unit with recent crater ejecta and small impacts suggests that its exposure or formation is younger than that of the dark terrain, and that it may be a result of the cratering or ejecta emplacement process itself. Although delineation of the lowland bright and dark subunits is limited to the box 1 valley region in this paper, the light and dark surface types are evident throughout the three candidate sites superposed on other mapped units (see Figure 5) Crater Interior and Crater Ejecta [21] Distributed across all other units are the crater interior and crater ejecta units (Ci, Ce; see Figure 5d). The crater interior unit is defined by the presence of a primary bowl-shaped or circular impact structure. Larger craters are discerned by topographic expression aided by albedo and illumination effects, while smaller craters are identified by the presence of circular albedo features alone. The ejecta unit consists of material emplaced during the impact process, and is characterized by radial to braided distribution patterns as well as circumferential topographic features such as ramparts. Overall, craters in the candidate landing sites appear severely degraded, are often pedestal [Barlow et al., 2000], and ejecta deposits are rarely evident. It is also common for small (<1 km diameter) degraded craters to be filled, appear level with the surrounding landscape, and exhibit bright crater rims instead of dark reminiscent of the lowland bright and dark terrains. Rare small fresh craters (generally <100 m diameter) with sharp rims do not exhibit these inverted albedo characteristics, and when present, have dark radial ejecta blankets. These observations are consistent with erosional processes acting to deflate the surface over time, locally exhuming and/or redepositing material. Large craters that are more pristine in morphologic appearance (e.g., Heimdall) tend to exhibit a distal halo of relatively high albedo terrain that is mapped to lowland bright terrain. Similar albedo patterns have been observed in association with lunar impacts, and are thought to be due to the emplacement of a fine ejecta layer [Ghent et al., 2005]. If this process is also occurring on Mars, deposition of fine-grained material could substantially affect the near-surface thermal regime [Sizemore and Mellon, 7of16

8 Figure 5. Type localities for geomorphic units. THEMIS visible image mosaics and profiles from MOLA 256 pixels/degree gridded data are shown for each area; variable vertical exaggeration is noted at top center of each profile. Figures 2 4 indicate context. (a) Blocks/mesas are large, flat-topped positive topographic features sometimes with raised margins. (b) Hills and knobs of the knobs unit typically have several hundred meters of relief and are commonly located beside topographic depressions (lowland) or elevated plateaus (blocks/mesas). Solitary hills have also been identified (an example is shown in Figure 8). (c) The lowland unit comprises wide, shallow, and typically enclosed topographic depressions. Some also exhibit elevated rims with respect to the surrounding plains. (d) Crater interior and ejecta. As in this example, many craters display rampart ejecta morphology. Crater interiors often have N-S asymmetry and floors that lie above the elevation of the surrounding plains. 8of16

9 2006]. A change in ice table depth or degree of ice cementation could affect the relative rates of aeolian erosion and lead to differential erosion of adjacent surfaces. 4. Spectral Characterization 4.1. Data Set and Method Description [22] Surface spectral variability was investigated through analysis of CRISM data. The CRISM instrument is a hyperspectral imaging spectrometer with 544 bands in the visible/ near infrared wavelength range from to 3.92 mm [Murchie et al., 2007]. The instrument consists of two detectors, VNIR (or shortwave, S) and IR (or longwave, L) that overlap near 1 mm. CRISM has two primary operational modes, push-broom multispectral and gimbaled hyperspectral, that allow for both global multispectral coverage at reduced spatial resolution and targeted hyperspectral coverage of interesting areas where higher spatial resolution and spectral sampling are advantageous. The multispectral mode acquires 100 or 200 m/pixel observations in 10-km-wide strips with 72 select channels from 0.41 to 3.92 mm. The hyperspectral targeted mode provides 20 or 40 m/pixel observations with roughly km or km footprints, respectively. The targeted mode is achieved by utilizing a pivot to compensate for spacecraft motion, which in addition allows for the acquisition of incoming and outgoing emission phase function observations. The motion of the gimbal also gives map-projected targeted images a characteristic hourglass shape. [23] The Phoenix landing site support strategy for MRO has been to provide coordinated CRISM multispectral mapping (MSP) coverage with every HiRISE image acquired within the landing region, and hyperspectral fullresolution targeted (FRT) CRISM observations for select areas usually near the centers of the candidate landing sites [Seelos et al., 2007]. Prior to the onset of unfavorable illumination and atmospheric conditions in the northern fall and winter, CRISM was able to acquire at least three FRT observations in each of the three landing site boxes under consideration. One FRT per box was selected for detailed spectral analysis on the basis of atmospheric clarity, illumination conditions, and represented terrain types. The three selected FRT images sample a majority of the mapped geomorphic units: lowland bright, lowland dark, and crater interior in box 1, crater interior, crater ejecta, highland, and knob in box 2, and highland, crater interior and crater ejecta in box 3. [24] Spectral radiance measured by CRISM is influenced by both surface reflectance and atmospheric radiative transfer. To correct for atmospheric scattering and absorption and retrieve surface Lambert albedo spectra, the FRT data were modeled using the Discrete Ordinate Radiative Transfer (DISORT) modeling code [Stamnes et al., 1988]. The DISORT software models gas absorptions (CO 2, CO, H 2 O), and scattering and absorption resulting from dust and ice aerosols, along with surface bidirectional reflectance and surface and atmospheric emission. [25] Model inputs specifying the state of the atmosphere include an atmospheric pressure and temperature profile, the water vapor column abundance, and dust and ice aerosol optical depths appropriate for the time and location of each CRISM observation. In addition, observation specific illumination and viewing geometries and the nominal passband centers and full width at half maximum (FWHM) values for the CRISM S and L detectors are provided. The model is initialized with a multilayer plane parallel representation of the atmosphere constructed by Runge-Kutta integration of the equation of hydrostatic equilibrium. For each CRISM passband, the dust single scattering albedo and scaled optical depth is calculated from the particle size distribution (nominal dust loading) and indices of refraction from Wolff and Clancy [2003], which incorporates laboratory measurements of dehydrated palagonite in the near-infrared. The aerosol scattering phase function was adopted from Clancy et al. [2003]. Molecular absorption parameters were calculated using the correlated-k approach with HITRAN line parameters. [26] Preliminary model runs were conducted to optimize the observation wavelength specification by minimizing residuals in the 2 mm wavelength region. An atmospheric dust optical depth of 0.3 at a reference wavelength of 9.3 mm and an ice aerosol opacity of 0.03 at 12.1 mm provided good retrievals as did historical climatology-based atmospheric pressure and temperature profiles adjusted via pressure scaling to minimize residual CO 2 bands [e.g., Smith, 2004]. Once the atmospheric model parameters have been established, the system is used to generate a multidimensional look up table that relates observed apparent I/F to surface Lambert albedo for each CRISM passband. The retrieval of surface Lambert albedo spectra is then accomplished through reverse lookup and linear interpolation of table elements. [27] Lambert albedo retrievals for MSP data used climatologically derived atmospheric parameter values without iteration [McGuire et al., 2008]. Because of the rapidly changing atmospheric conditions relative to the historical climatology tables, significant residual variability can be apparent between adjacent images. [28] Interpretation of model Lambert albedo CRISM FRT and MSP spectra was performed through qualitative inspection of summary parameter maps [Pelkey et al., 2007] and comparison to both library mineral spectra and an analog surface in Gusev crater where landed observations are also available. Summary parameter maps depict the spatial distribution of minerals that have diagnostic features, such as olivine, pyroxene, sulfates, and phyllosilicates, as well as H 2 O and CO 2 surface ices. The MSP data set and resulting products were examined for coherent spectral variability at the 0.5 km spatial scale, while FRT data provide resolvability to 50 m. Since the CRISM FRT data adequately sample the surface types represented in the candidate landing sites and are of higher spatial and spectral quality than the MSP data, they are the focus of the spectral analysis presented here. For comparative purposes, FRT spectra were extracted from end-member locations in each scene, with each spectrum being an average of a 3 3 pixel kernel. [29] End-member spectra with well-defined absorption features allow for a quantitative assessment of the spectral information via a bidirectional Hapke model [Hapke, 1993]. The spectra were modeled as an intimate mixture of four potential components chosen as likely constituents after initial inspection: water ice, basalt, olivine (Fo 88.5 ), and palagonite (a weathering product of basaltic glass as a proxy for Martian dust). The water ice optical constants are 9of16

10 Table 1. CRISM Spectral Sampling Summary Landing Site, CRISM Image ID Label Geomorphic Unit a Surface Description Box 1 and FRT ; see Figures 6 and 7 Box 2 and FRT ; see Figures 8 and 9 Box 3 and FRT C; see Figures 10 and 11 a See Figures 2 4. b See section 4.2. Infrared Spectral Slope Category b A crater interior low albedo rim, boulders neutral B crater interior bright, dunes positive C lowland bright intercrater plains, polygons, few rocks neutral D lowland dark intercrater plains, polygons, some rock clusters neutral A crater interior bright, dunes positive B crater ejecta relatively dark, polygons, some rocks neutral C crater ejecta intermediate, large bright polygonal cracks neutral A highland intercrater plains, polygons, apparent infill positive of bright material, few rocks B highland intercrater plains, polygons, few rocks neutral C crater interior bright north-facing deposit negative combined from Warren [1984] and Grundy and Schmitt [1998]. Basalt optical constants are from Pollack et al. [1973]. Olivine optical constants were provided by P. Lucey (personal communication, 2007). Palagonite optical constants are a revised version of the data presented in the work of Clancy et al. [1995]. [30] Hapke s [1993] equation 10.4 is used to calculate the Lambert reflectance. The equation is: ri; ð e; g; l; wþ ¼ w 4 1 ðm O þ mþ f½1 þ Bg ð ÞŠPg ð ÞþH ð m O ÞHðmÞ 1g 4.2. Results [32] Hyperspectral CRISM data sample a variety of terrains found in the three candidate sites (Table 1). Box 1 CRISM data highlight the lowland bright and lowland dark geomorphic units that dominate the valley of the primary landing ellipse (Figure 6). Small craters with high albedo interior deposits and degraded craters with low albedo rims are also evident. The spectra extracted for these four surface types are shown in Figure 7 along with HiRISE data to where r is reflectance at a specific wavelength (l), i, e, and g are the incidence, emission and phase angles, m = cos(e), m 0 = cos(i), w is the single scattering albedo, P(g) is the scattering phase function, B(g) is the backscattering function, and H is Chandresekhar s H-function for isotropic scatters [Chandresekhar, 1960]. Although the observation geometry of the CRISM data under consideration did not approach zero phase, the model includes an approximation for the backscattering function given by Hapke [1993, equation 8.90]: B O Bg ð Þ ¼ 1 þ ð1=hþtanðg=2þ where B o is given by Hapke [1993, equation 8.86], B o = S(0)/wP(0), where S(0) is the light scattered at, or close to the part of the particulate surface facing the source. We use h = 0.05 (a lunar-like surface). Although the software is designed to use a two term Legendre polynomial to represent P(g), here isotropic scattering is assumed, and hence P(g) = 1. The optical constants are used to calculate w as described in the work of Roush [1994]. [31] The modeling included several mixtures of the candidate materials, but we did not investigate all possible combinations and permutations. In addition, only a single grain size for each component in the mixture model was considered at each iteration. The model component grain sizes and relative mass fractions were allowed to vary freely until a nonlinear least squares fit between the observed and calculated spectrum was minimized using the reduced c 2 value [Press et al., 1992]. The curve fitting routine relied upon a downhill simplex method described by Press et al. [1992]. Figure 6. Lowland bright and dark terrains in box 1 near the primary landing location. CRISM image FRT is shown as an enhanced visible composite (R, 0.71 mm; G, 0.62 mm; B, 0.56 mm) overlain on co-temporal CTX data (image P02_001959_2484_XI_68N127W_061227; see Figure 2 for placement). Lowland bright terrain dominates the eastern portion of the area and appears smoother with an overall higher albedo. Lowland dark terrain is observed primarily in the south and west, exhibits lower albedo, and is elevated with respect to the lowland bright terrain by a few meters (visual estimation). Owing to the late northern summertime of acquisition (Ls = ), water ice clouds are apparent as bluish haze in the CRISM data. HiRISE subsets shown in Figure 7 are labeled A D. Illumination is from the lower left with an incidence angle of of 16

11 are extracted from plains locations and rocky crater rims in box 1 (Figures 7a, 7c, and 7d), crater ejecta in box 2 (Figures 9b and 9c), and relatively dark plains in box 3 (Figure 11b). These spectra exhibit a ferric absorption edge in the visible wavelength range indicative of nanophase iron oxides, as well as broad shallow absorptions near 1 and 2 mm that suggest basaltic mineralogy (i.e., pyroxene and olivine). Bands near 1.9 mm and a roll-off longward of 2.4 mm may be features of uncorrected residual atmospheric ices, or small amounts of adsorbed water [Milliken et al., 2007]. Differences in overall albedo between these materials may be explained by relative abundances and spatial mixing of rocks and dust as observed in the high-resolution HiRISE data. [34] Spectra with a slightly positive infrared slope are extracted from bright deposits inside small craters, exemplified in boxes 1 and 2 (Figures 7b and 9a), as well as the mottled plains in box 3 where bright dust appears to have preferentially settled within polygon troughs (Figure 11a). The crater floor deposits exhibit meter-scale dunes, implying that these low areas serve as traps for windblown materials. The positive infrared spectral slope is consistent with an optically thick deposit of fine-grained ferric-rich particulates [Fischer and Pieters, 1993], but otherwise similar spectral features to that of the previous category implies the same constituent mineralogy. [35] The third spectral category is represented by a sample of high albedo material on the north-facing slopes of a small fresh crater in box 3 (Figure 11c). This negatively Figure 7. HiRISE terrain samples and corresponding CRISM Lambert albedo spectra for areas indicated in Figure 6. (a) A degraded crater rim exhibits rocks and boulders, at times several meters in diameter. The CRISM spectrum shows a relatively dark surface with subtle basaltic and dust signatures. (b) Rippled aeolian deposits fill a small crater and have an elevated albedo and positive infrared spectral slope. (c) Lowland bright terrain exhibits ubiquitous meter-scale polygonal ground and sparse rocks; the shape of the spectrum is similar to that of Figure 7a but with slightly enhanced overall albedo. (d) Lowland dark terrain is likewise dominated by polygonal ground, but frequent boulder clusters contribute to its darker albedo. HiRISE image PSP_001959_2485 is shown. illustrate the fine-scale surface properties. The box 2 CRISM FRT is positioned on the edge of a large rampart ejecta deposit (Figure 8). Three spectra selected for analysis in this image include the relatively bright ejecta rampart, relatively dark plains, and a high albedo deposit within a small crater (Figure 9). Finally, box 3 CRISM data show mottled plains with a fairly fresh, small impact crater exhibiting dark ejecta and bright interior deposit (Figure 10). Spectra of the bright and dark plains as well as the crater s bright interior deposit were chosen for evaluation in this scene (Figure 11). [33] Examination of the end-member spectra reveal that they can be grouped into three categories on the basis of infrared spectral slope from approximately 1 to 2 mm: neutral, positive, and negative. The neutral slope spectra Figure 8. Rampart crater ejecta and plains materials in box 2. CRISM image FRT is shown as an enhanced visible composite (R, 0.71 mm; G, 0.62 mm; B, 0.56 mm) overlain on cotemporal CTX data (image P02_001853_2468_XI_66N112W_061218; see Figure 3 for placement). The distal margin of rampart ejecta is observed in the north and east, while smooth plains (highland unit) dominate the west. A symmetric hill (mapped as the knobs unit) is also evident. HiRISE subsets shown in Figure 9 are labeled A C. Illumination is from the lower left with an incidence angle of 64, and the images were acquired at a solar longitude of of 16

12 Figure 9. HiRISE terrain samples and corresponding CRISM Lambert albedo spectra for areas indicated in Figure 8. (a) A small, fairly recent impact exhibits large boulders along the rim as well as a bright interior deposit with aeolian dunes. Like the similar bright crater deposit in Figure 7b, the CRISM spectrum shows elevated albedo and positive infrared spectral slope. (b) Relatively dark radial ejecta from the crater shown in Figure 9a is composed of meter-scale polygonal ground with widely dispersed boulders. (c) Moderate albedo rampart ejecta also exhibits polygonal ground with long, light-toned sinuous troughs. HiRISE image PSP_001853_2470 is shown. shown in the hyperspectral data; nanophase iron oxides and subtle basaltic signatures dominate the region. There is no indication of sulfates or other cation-hydroxyl-bearing minerals within the first few tens of microns of the surface. However, kilometer-wide deposits of water ice are apparent on some north-facing slopes of crater walls and escarpments, for example in Heimdall crater in box 1 (Figure 13). A MOC image of the Heimdall deposit acquired at comparable solar longitude (L s ) shows that it is patchy in appearance and concentrated along shadowed ledges. The late summer presence of ice deposits suggests that they are persistent all year long; indeed, water ice outliers are commonly observed below 70 N throughout the northern plains [Seelos et al., 2008]. Because these deposits would continue to sublimate and shrink in size throughout the summer, it is likely that they are larger and that additional ice deposits are present earlier in the season over the time period that Phoenix will be performing surface operations (Ls ) [see Arvidson et al., 2008] Comparison to Gusev Crater Plains [37] The in situ observations made by prior landed missions may provide insight into the landscape that Phoenix will encounter. While the Viking 2 landing site in eastern Utopia Planitia is located in a similar geologic setting sloped spectrum exhibits strong absorptions centered at 1.5 mm and 2.0 mm with a smaller feature at 1.25 mm and drop-off longward of 2.4 mm. These features are diagnostic of water ice, although spectral characteristics of nanophase iron oxides in the visible wavelength range indicate that the water ice is not pure and/or spatially uniform. Initial intimate mixture modeling of the infrared (L detector) portion of the spectrum show the best-fit components to be 57% (relative mass fraction) water ice and 43% palagonite, with grain sizes of 0.08 and mm, respectively (Table 2 and Figure 12). The nonlinear least squares fit using these model parameters produces a reduced chisquare of Other models utilizing water ice with either basalt or a combination of basalt, olivine, and palagonite also resulted in low reduced chi-square values of and 0.032, respectively, but have optimized grain sizes for the basalt component that are in violation of the geometric optics assumption. Model fits that included no water ice at all had systematically larger error, underscoring the significance of the water ice model component. Poor model fits suggest that additional, alternate, or different combinations of components are required. [36] Evaluation of CRISM MSP data and summary parameter maps yield similar spectroscopic signatures to those Figure 10. Mottled plains and fresh impact crater in box 3. CRISM image FRT C is shown as an enhanced visible composite (R, 0.71 mm; G, 0.62 mm; B, 0.56 mm) overlain on cotemporal CTX data (image P01_001602_ 2511_XI_71N106W_061129; see Figure 4 for placement). Lower albedo ejecta is observed along the eastern side of the image set, while light-and dark-toned plains make up a majority of the scene. A small crater with very dark ejecta and water ice deposits is shown enlarged in Figure 11c. Labels A and B denote HiRISE subsets shown in Figure 11. Illumination is from the lower left with an incidence angle of 62, and the images were acquired at a solar longitude of of 16

13 The visible absorption edge near 0.5 mm and peak at 0.7 mm are consistent with nanophase iron oxides. The Gusev plains spectra also exhibit a neutral infrared slope comparable to the dominant spectral type in the Phoenix observations. Unlike the Phoenix observations, no water is evident in the Gusev spectra, which is not unexpected given the dry nearequatorial environment. On the basis of the similarity of spectral characteristics between the Gusev plains and the Phoenix landing sites, analogous surface mineralogy is a reasonable expectation. Figure 11. Terrain samples from HiRISE and CRISM and corresponding CRISM Lambert albedo spectra for areas indicated in Figure 10. (a) Light-toned plains and (b) comparably dark-toned plains are dominated by meter-scale polygonal ground with few rocks. HiRISE image PSP_001602_2510 is shown. (c) Enlargement of CRISM RGB composite from Figure 10 highlights a fresh impact crater with water ice deposits on north-facing slopes (denoted by light blue color and arrows). [Mutch et al., 1977; Arvidson et al., 1989], the robust mineralogical and compositional characterization of the Mars Exploration Rover (MER) Spirit landing site in Gusev crater provides a more accurate depiction of what may control the spectral properties of the candidate Phoenix landing sites. Panchromatic Camera (Pancam) observations show the plains surrounding the Spirit landing site (14.6 S, E) to be rocky with thin deposits of bright dust and duricrust materials [e.g., Bell et al., 2004; Arvidson et al., 2006; Golombek et al., 2006] (Figure 14). Where the rover has traversed across the plains and disturbed the surface layer, a darker underlying soil is exposed. These materials were measured by the Mini-Thermal Emission Spectrometer (Mini-TES) and Mössbauer instruments and found to be of basaltic composition, containing olivine, pyroxene, magnetite, and nanophase iron oxides [Christensen et al., 2004b; Yen et al., 2005; Morris et al., 2006]. Orbital visible/near infrared data from the Observatoire pour la Minéralogie l Eau, les Glaces et l Activité (OMEGA) instrument are consistent with the landed observations and indicate that the plains are dominated by the spectral signature of dust and weakly altered basaltic sands [Lichtenberg et al., 2007]. [38] CRISM FRT data acquired of the Spirit landing site at Gusev were processed in the same manner as that of the Phoenix FRT images; representative spectra are displayed in Figure 14. Broad, shallow absorptions near 1 and 2 mm are indicative of the basaltic components olivine and pyroxene. 5. Synthesis [39] The three candidate landing site boxes sample broadly similar northern plains landscapes along a diffuse geologic boundary between the Amazonian-aged Scandia region and Vastitas Borealis marginal units [Tanaka et al., 2005]. Box 1 straddles the boundary, box 2 is located primarily on Vastitas Borealis marginal unit, and box 3 is entirely within the Scandia region unit. The 150-to 200-kmwide zone between these geologic units contains several distinct geomorphic units that are suggestive of a high degree of modification. To the south, the relatively smooth intercrater plains are punctuated by occasional irregular depressions and upraised mesas, both commonly rimmed with knob terrain. The knobs unit occupies a wide swath along the transition zone, followed by the disrupted blocks/ mesas unit and coalesced lowland depressions. While the lowland depressions would seem to imply erosion or subsidence, the upraised knob terrain may be consistent with localized deposition or the remains of a once more extensive layer. Knobs may also be the tops of buried landforms. Interlocking blocks/mesas along the transition zone are similar in appearance to the highland unit, perhaps implying that the highland unit (and Vastitas Borealis marginal unit) was once more extensive and was disrupted and/or eroded along this boundary. The depressions, knobs, and low mesas located well away from the boundary may also indicate incipient areas of modification and/or deposition. The nature and association of the geomorphic units are consistent with the hypotheses proposed by Tanaka et al. [2005] that subsurface volatiles, perhaps cryovolcanism, or magmatic activity was responsible for the diversity of landforms. [40] CRISM observations demonstrate that the spectral character of the candidate landing sites is decoupled from the geomorphologic variability. Any mineralogic evidence Table 2. Hapke Model Parameters for Box 3C Spectral End- Member Model Water Ice a Basalt a Olivine a Palagonite a Reduced c 2 m1a 45: <1: : : m2a 69: : : m3a 13: : m4a 27: : m5a 86: : m6a <1: : m7a 57: : a Relative mass fraction (%) and grain diameter (mm) are provided for each component; values in italics represent grain sizes that violate the underlying geometric optics assumption inherent in the model. 13 of 16

14 Figure 12. Hapke model fits to the box 3C spectral end-member (Figure 11). The best fitting models m1a, m5a, and m7a incorporate water ice as a primary component. Table 2 tabulates the model parameters and results. of the events that transpired to form the geomorphologic units, if ever present, has since been obscured by more recent surface activity. Surface materials comprising the lowland dark and lowland bright terrains of the primary candidate landing site also dominate the two alternate sites irrespective of the geomorphic or geologic unit boundaries. Similar mineralogy is evident at all three sites, and is inferred to primarily consist of fine-grained nanophase iron oxides with basaltic sand and rocks. Whether this surficial material was deposited as a widespread thin mantle, perhaps akin to midlatitude mantle deposits observed by Mustard et al. [2001], or resulted from local physical reworking of preexisting sediments, it serves to mask any possible record of chemical alteration that may have provided insight into Figure 13. Water ice deposits in Heimdall crater observed in CRISM MSP data. (a) Composite of MOC (M , E ) and THEMIS visible (V ) data of Heimdall crater with CRISM overlay of 1500-nm band depth. CRISM band depth at 1500 nm is indicative of water ice, shown colorized from dark blue (low) to light blue (high); water ice deposits on the north-facing shadowed slope of the crater wall and a nearby escarpment are evident (arrows). The white box indicates location of Figure 13b. (b) Enlargement of MOC image E reveals the patchy appearance of the water ice in Heimdall crater at a comparable season to that of the CRISM data (Ls = and ). Deformation of crater floor materials is also apparent, perhaps suggestive of periglacial processes. MOC image illumination is from the lower left. 14 of 16

15 Mars and its capacity, past and present, for habitable environments. [42] Acknowledgments. This research was supported by NASA through the Phoenix and MRO/CRISM Missions. Many thanks are due to the CRISM, HiRISE, CTX, THEMIS, and MOC instrument teams for their continued support of the Phoenix landing site characterization effort. The authors would also like to thank J. Skinner and K. Tanaka for thoughtful review and discussion. Figure 14. Gusev crater plains near the MER Spirit landing site. (top) A portion of the Legacy panorama acquired by the Pancam instrument shows that the Gusev plains are dominated by scattered boulders and bright surface materials that overlie a darker substrate, observed when disturbed by the rover wheels (right edge of image). The Legacy panorama was acquired on sols 59 61, 3 6 March Image courtesy of NASA/JPL/Cornell University. (bottom) Select CRISM spectra of the Gusev plains are similar in overall albedo and shape to the spectra acquired of the candidate Phoenix landing sites. past geological processes. Physical weathering now appears to prevail in the form of aeolian and ice-driven processes. Windblown dust preferentially collects in low areas, and the common observation of filled, degraded craters suggests that aeolian activity has been a dominant and persistent surface modification process. The ubiquitous presence of decameter-scale polygonal terrain [e.g., Mellon et al., 2008] also suggests the active and pervasive influence of subsurface water ice. [41] The combined evaluation of geomorphic and mineralogic characteristics of the candidate Phoenix landing sites supports a complex Amazonian-aged geologic history overprinted by modern surface processes. The Phoenix mission would likely find similar materials, compositionally and physically, at all three candidate landing sites. 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